"""
Least Angle Regression algorithm. See the documentation on the
Generalized Linear Model for a complete discussion.
"""
# Author: Fabian Pedregosa <fabian.pedregosa@inria.fr>
#         Alexandre Gramfort <alexandre.gramfort@inria.fr>
#         Gael Varoquaux
#
# License: BSD 3 clause

from math import log
import sys
import warnings

import numpy as np
from scipy import linalg, interpolate
from scipy.linalg.lapack import get_lapack_funcs
from joblib import Parallel, delayed

from ._base import LinearModel
from ..base import RegressorMixin, MultiOutputMixin
# mypy error: Module 'sklearn.utils' has no attribute 'arrayfuncs'
from ..utils import arrayfuncs, as_float_array  # type: ignore
from ..utils import check_random_state
from ..model_selection import check_cv
from ..exceptions import ConvergenceWarning
from ..utils.validation import _deprecate_positional_args

SOLVE_TRIANGULAR_ARGS = {'check_finite': False}


@_deprecate_positional_args
def lars_path(X, y, Xy=None, *, Gram=None, max_iter=500, alpha_min=0,
              method='lar', copy_X=True, eps=np.finfo(np.float).eps,
              copy_Gram=True, verbose=0, return_path=True,
              return_n_iter=False, positive=False):
    """Compute Least Angle Regression or Lasso path using LARS algorithm [1]

    The optimization objective for the case method='lasso' is::

    (1 / (2 * n_samples)) * ||y - Xw||^2_2 + alpha * ||w||_1

    in the case of method='lars', the objective function is only known in
    the form of an implicit equation (see discussion in [1])

    Read more in the :ref:`User Guide <least_angle_regression>`.

    Parameters
    ----------
    X : None or array-like of shape (n_samples, n_features)
        Input data. Note that if X is None then the Gram matrix must be
        specified, i.e., cannot be None or False.

    y : None or array-like of shape (n_samples,)
        Input targets.

    Xy : array-like of shape (n_samples,) or (n_samples, n_targets), \
            default=None
        Xy = np.dot(X.T, y) that can be precomputed. It is useful
        only when the Gram matrix is precomputed.

    Gram : None, 'auto', array-like of shape (n_features, n_features), \
            default=None
        Precomputed Gram matrix (X' * X), if ``'auto'``, the Gram
        matrix is precomputed from the given X, if there are more samples
        than features.

    max_iter : int, default=500
        Maximum number of iterations to perform, set to infinity for no limit.

    alpha_min : float, default=0
        Minimum correlation along the path. It corresponds to the
        regularization parameter alpha parameter in the Lasso.

    method : {'lar', 'lasso'}, default='lar'
        Specifies the returned model. Select ``'lar'`` for Least Angle
        Regression, ``'lasso'`` for the Lasso.

    copy_X : bool, default=True
        If ``False``, ``X`` is overwritten.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. By default, ``np.finfo(np.float).eps`` is used.

    copy_Gram : bool, default=True
        If ``False``, ``Gram`` is overwritten.

    verbose : int, default=0
        Controls output verbosity.

    return_path : bool, default=True
        If ``return_path==True`` returns the entire path, else returns only the
        last point of the path.

    return_n_iter : bool, default=False
        Whether to return the number of iterations.

    positive : bool, default=False
        Restrict coefficients to be >= 0.
        This option is only allowed with method 'lasso'. Note that the model
        coefficients will not converge to the ordinary-least-squares solution
        for small values of alpha. Only coefficients up to the smallest alpha
        value (``alphas_[alphas_ > 0.].min()`` when fit_path=True) reached by
        the stepwise Lars-Lasso algorithm are typically in congruence with the
        solution of the coordinate descent lasso_path function.

    Returns
    -------
    alphas : array-like of shape (n_alphas + 1,)
        Maximum of covariances (in absolute value) at each iteration.
        ``n_alphas`` is either ``max_iter``, ``n_features`` or the
        number of nodes in the path with ``alpha >= alpha_min``, whichever
        is smaller.

    active : array-like of shape (n_alphas,)
        Indices of active variables at the end of the path.

    coefs : array-like of shape (n_features, n_alphas + 1)
        Coefficients along the path

    n_iter : int
        Number of iterations run. Returned only if return_n_iter is set
        to True.

    See also
    --------
    lars_path_gram
    lasso_path
    lasso_path_gram
    LassoLars
    Lars
    LassoLarsCV
    LarsCV
    sklearn.decomposition.sparse_encode

    References
    ----------
    .. [1] "Least Angle Regression", Efron et al.
           http://statweb.stanford.edu/~tibs/ftp/lars.pdf

    .. [2] `Wikipedia entry on the Least-angle regression
           <https://en.wikipedia.org/wiki/Least-angle_regression>`_

    .. [3] `Wikipedia entry on the Lasso
           <https://en.wikipedia.org/wiki/Lasso_(statistics)>`_

    """
    if X is None and Gram is not None:
        raise ValueError(
            'X cannot be None if Gram is not None'
            'Use lars_path_gram to avoid passing X and y.'
        )
    return _lars_path_solver(
        X=X, y=y, Xy=Xy, Gram=Gram, n_samples=None, max_iter=max_iter,
        alpha_min=alpha_min, method=method, copy_X=copy_X,
        eps=eps, copy_Gram=copy_Gram, verbose=verbose, return_path=return_path,
        return_n_iter=return_n_iter, positive=positive)


@_deprecate_positional_args
def lars_path_gram(Xy, Gram, *, n_samples, max_iter=500, alpha_min=0,
                   method='lar', copy_X=True, eps=np.finfo(np.float).eps,
                   copy_Gram=True, verbose=0, return_path=True,
                   return_n_iter=False, positive=False):
    """lars_path in the sufficient stats mode [1]

    The optimization objective for the case method='lasso' is::

    (1 / (2 * n_samples)) * ||y - Xw||^2_2 + alpha * ||w||_1

    in the case of method='lars', the objective function is only known in
    the form of an implicit equation (see discussion in [1])

    Read more in the :ref:`User Guide <least_angle_regression>`.

    Parameters
    ----------
    Xy : array-like of shape (n_samples,) or (n_samples, n_targets)
        Xy = np.dot(X.T, y).

    Gram : array-like of shape (n_features, n_features)
        Gram = np.dot(X.T * X).

    n_samples : int or float
        Equivalent size of sample.

    max_iter : int, default=500
        Maximum number of iterations to perform, set to infinity for no limit.

    alpha_min : float, default=0
        Minimum correlation along the path. It corresponds to the
        regularization parameter alpha parameter in the Lasso.

    method : {'lar', 'lasso'}, default='lar'
        Specifies the returned model. Select ``'lar'`` for Least Angle
        Regression, ``'lasso'`` for the Lasso.

    copy_X : bool, default=True
        If ``False``, ``X`` is overwritten.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. By default, ``np.finfo(np.float).eps`` is used.

    copy_Gram : bool, default=True
        If ``False``, ``Gram`` is overwritten.

    verbose : int, default=0
        Controls output verbosity.

    return_path : bool, default=True
        If ``return_path==True`` returns the entire path, else returns only the
        last point of the path.

    return_n_iter : bool, default=False
        Whether to return the number of iterations.

    positive : bool, default=False
        Restrict coefficients to be >= 0.
        This option is only allowed with method 'lasso'. Note that the model
        coefficients will not converge to the ordinary-least-squares solution
        for small values of alpha. Only coefficients up to the smallest alpha
        value (``alphas_[alphas_ > 0.].min()`` when fit_path=True) reached by
        the stepwise Lars-Lasso algorithm are typically in congruence with the
        solution of the coordinate descent lasso_path function.

    Returns
    -------
    alphas : array-like of shape (n_alphas + 1,)
        Maximum of covariances (in absolute value) at each iteration.
        ``n_alphas`` is either ``max_iter``, ``n_features`` or the
        number of nodes in the path with ``alpha >= alpha_min``, whichever
        is smaller.

    active : array-like of shape (n_alphas,)
        Indices of active variables at the end of the path.

    coefs : array-like of shape (n_features, n_alphas + 1)
        Coefficients along the path

    n_iter : int
        Number of iterations run. Returned only if return_n_iter is set
        to True.

    See also
    --------
    lars_path
    lasso_path
    lasso_path_gram
    LassoLars
    Lars
    LassoLarsCV
    LarsCV
    sklearn.decomposition.sparse_encode

    References
    ----------
    .. [1] "Least Angle Regression", Efron et al.
           http://statweb.stanford.edu/~tibs/ftp/lars.pdf

    .. [2] `Wikipedia entry on the Least-angle regression
           <https://en.wikipedia.org/wiki/Least-angle_regression>`_

    .. [3] `Wikipedia entry on the Lasso
           <https://en.wikipedia.org/wiki/Lasso_(statistics)>`_

    """
    return _lars_path_solver(
        X=None, y=None, Xy=Xy, Gram=Gram, n_samples=n_samples,
        max_iter=max_iter, alpha_min=alpha_min, method=method,
        copy_X=copy_X, eps=eps, copy_Gram=copy_Gram,
        verbose=verbose, return_path=return_path,
        return_n_iter=return_n_iter, positive=positive)


def _lars_path_solver(X, y, Xy=None, Gram=None, n_samples=None, max_iter=500,
                      alpha_min=0, method='lar', copy_X=True,
                      eps=np.finfo(np.float).eps, copy_Gram=True, verbose=0,
                      return_path=True, return_n_iter=False, positive=False):
    """Compute Least Angle Regression or Lasso path using LARS algorithm [1]

    The optimization objective for the case method='lasso' is::

    (1 / (2 * n_samples)) * ||y - Xw||^2_2 + alpha * ||w||_1

    in the case of method='lars', the objective function is only known in
    the form of an implicit equation (see discussion in [1])

    Read more in the :ref:`User Guide <least_angle_regression>`.

    Parameters
    ----------
    X : None or ndarray, of shape (n_samples, n_features)
        Input data. Note that if X is None then Gram must be specified,
        i.e., cannot be None or False.

    y : None or ndarray, of shape (n_samples,)
        Input targets.

    Xy : array-like of shape (n_samples,) or (n_samples, n_targets), \
            default=None
        Xy = np.dot(X.T, y) that can be precomputed. It is useful
        only when the Gram matrix is precomputed.

    Gram : None, 'auto' or array-like of shape (n_features, n_features), \
            default=None
        Precomputed Gram matrix (X' * X), if ``'auto'``, the Gram
        matrix is precomputed from the given X, if there are more samples
        than features.

    n_samples : int or float, default=None
        Equivalent size of sample.

    max_iter : int, default=500
        Maximum number of iterations to perform, set to infinity for no limit.

    alpha_min : float, default=0
        Minimum correlation along the path. It corresponds to the
        regularization parameter alpha parameter in the Lasso.

    method : {'lar', 'lasso'}, default='lar'
        Specifies the returned model. Select ``'lar'`` for Least Angle
        Regression, ``'lasso'`` for the Lasso.

    copy_X : bool, default=True
        If ``False``, ``X`` is overwritten.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. By default, ``np.finfo(np.float).eps`` is used

    copy_Gram : bool, default=True
        If ``False``, ``Gram`` is overwritten.

    verbose : int, default=0
        Controls output verbosity.

    return_path : bool, default=True
        If ``return_path==True`` returns the entire path, else returns only the
        last point of the path.

    return_n_iter : bool, default=False
        Whether to return the number of iterations.

    positive : bool, default=False
        Restrict coefficients to be >= 0.
        This option is only allowed with method 'lasso'. Note that the model
        coefficients will not converge to the ordinary-least-squares solution
        for small values of alpha. Only coefficients up to the smallest alpha
        value (``alphas_[alphas_ > 0.].min()`` when fit_path=True) reached by
        the stepwise Lars-Lasso algorithm are typically in congruence with the
        solution of the coordinate descent lasso_path function.

    Returns
    -------
    alphas : array-like of shape (n_alphas + 1,)
        Maximum of covariances (in absolute value) at each iteration.
        ``n_alphas`` is either ``max_iter``, ``n_features`` or the
        number of nodes in the path with ``alpha >= alpha_min``, whichever
        is smaller.

    active : array-like of shape (n_alphas,)
        Indices of active variables at the end of the path.

    coefs : array-like of shape (n_features, n_alphas + 1)
        Coefficients along the path

    n_iter : int
        Number of iterations run. Returned only if return_n_iter is set
        to True.

    See also
    --------
    lasso_path
    LassoLars
    Lars
    LassoLarsCV
    LarsCV
    sklearn.decomposition.sparse_encode

    References
    ----------
    .. [1] "Least Angle Regression", Efron et al.
           http://statweb.stanford.edu/~tibs/ftp/lars.pdf

    .. [2] `Wikipedia entry on the Least-angle regression
           <https://en.wikipedia.org/wiki/Least-angle_regression>`_

    .. [3] `Wikipedia entry on the Lasso
           <https://en.wikipedia.org/wiki/Lasso_(statistics)>`_

    """
    if method == 'lar' and positive:
        raise ValueError(
                "Positive constraint not supported for 'lar' "
                "coding method."
            )

    n_samples = n_samples if n_samples is not None else y.size

    if Xy is None:
        Cov = np.dot(X.T, y)
    else:
        Cov = Xy.copy()

    if Gram is None or Gram is False:
        Gram = None
        if X is None:
            raise ValueError('X and Gram cannot both be unspecified.')
        if copy_X:
            # force copy. setting the array to be fortran-ordered
            # speeds up the calculation of the (partial) Gram matrix
            # and allows to easily swap columns
            X = X.copy('F')

    elif isinstance(Gram, str) and Gram == 'auto' or Gram is True:
        if Gram is True or X.shape[0] > X.shape[1]:
            Gram = np.dot(X.T, X)
        else:
            Gram = None
    elif copy_Gram:
        Gram = Gram.copy()

    if Gram is None:
        n_features = X.shape[1]
    else:
        n_features = Cov.shape[0]
        if Gram.shape != (n_features, n_features):
            raise ValueError('The shapes of the inputs Gram and Xy'
                             ' do not match.')
    max_features = min(max_iter, n_features)

    if return_path:
        coefs = np.zeros((max_features + 1, n_features))
        alphas = np.zeros(max_features + 1)
    else:
        coef, prev_coef = np.zeros(n_features), np.zeros(n_features)
        alpha, prev_alpha = np.array([0.]), np.array([0.])  # better ideas?

    n_iter, n_active = 0, 0
    active, indices = list(), np.arange(n_features)
    # holds the sign of covariance
    sign_active = np.empty(max_features, dtype=np.int8)
    drop = False

    # will hold the cholesky factorization. Only lower part is
    # referenced.
    if Gram is None:
        L = np.empty((max_features, max_features), dtype=X.dtype)
        swap, nrm2 = linalg.get_blas_funcs(('swap', 'nrm2'), (X,))
    else:
        L = np.empty((max_features, max_features), dtype=Gram.dtype)
        swap, nrm2 = linalg.get_blas_funcs(('swap', 'nrm2'), (Cov,))
    solve_cholesky, = get_lapack_funcs(('potrs',), (L,))

    if verbose:
        if verbose > 1:
            print("Step\t\tAdded\t\tDropped\t\tActive set size\t\tC")
        else:
            sys.stdout.write('.')
            sys.stdout.flush()

    tiny32 = np.finfo(np.float32).tiny  # to avoid division by 0 warning
    equality_tolerance = np.finfo(np.float32).eps

    if Gram is not None:
        Gram_copy = Gram.copy()
        Cov_copy = Cov.copy()

    while True:
        if Cov.size:
            if positive:
                C_idx = np.argmax(Cov)
            else:
                C_idx = np.argmax(np.abs(Cov))

            C_ = Cov[C_idx]

            if positive:
                C = C_
            else:
                C = np.fabs(C_)
        else:
            C = 0.

        if return_path:
            alpha = alphas[n_iter, np.newaxis]
            coef = coefs[n_iter]
            prev_alpha = alphas[n_iter - 1, np.newaxis]
            prev_coef = coefs[n_iter - 1]

        alpha[0] = C / n_samples
        if alpha[0] <= alpha_min + equality_tolerance:  # early stopping
            if abs(alpha[0] - alpha_min) > equality_tolerance:
                # interpolation factor 0 <= ss < 1
                if n_iter > 0:
                    # In the first iteration, all alphas are zero, the formula
                    # below would make ss a NaN
                    ss = ((prev_alpha[0] - alpha_min) /
                          (prev_alpha[0] - alpha[0]))
                    coef[:] = prev_coef + ss * (coef - prev_coef)
                alpha[0] = alpha_min
            if return_path:
                coefs[n_iter] = coef
            break

        if n_iter >= max_iter or n_active >= n_features:
            break
        if not drop:

            ##########################################################
            # Append x_j to the Cholesky factorization of (Xa * Xa') #
            #                                                        #
            #            ( L   0 )                                   #
            #     L  ->  (       )  , where L * w = Xa' x_j          #
            #            ( w   z )    and z = ||x_j||                #
            #                                                        #
            ##########################################################

            if positive:
                sign_active[n_active] = np.ones_like(C_)
            else:
                sign_active[n_active] = np.sign(C_)
            m, n = n_active, C_idx + n_active

            Cov[C_idx], Cov[0] = swap(Cov[C_idx], Cov[0])
            indices[n], indices[m] = indices[m], indices[n]
            Cov_not_shortened = Cov
            Cov = Cov[1:]  # remove Cov[0]

            if Gram is None:
                X.T[n], X.T[m] = swap(X.T[n], X.T[m])
                c = nrm2(X.T[n_active]) ** 2
                L[n_active, :n_active] = \
                    np.dot(X.T[n_active], X.T[:n_active].T)
            else:
                # swap does only work inplace if matrix is fortran
                # contiguous ...
                Gram[m], Gram[n] = swap(Gram[m], Gram[n])
                Gram[:, m], Gram[:, n] = swap(Gram[:, m], Gram[:, n])
                c = Gram[n_active, n_active]
                L[n_active, :n_active] = Gram[n_active, :n_active]

            # Update the cholesky decomposition for the Gram matrix
            if n_active:
                linalg.solve_triangular(L[:n_active, :n_active],
                                        L[n_active, :n_active],
                                        trans=0, lower=1,
                                        overwrite_b=True,
                                        **SOLVE_TRIANGULAR_ARGS)

            v = np.dot(L[n_active, :n_active], L[n_active, :n_active])
            diag = max(np.sqrt(np.abs(c - v)), eps)
            L[n_active, n_active] = diag

            if diag < 1e-7:
                # The system is becoming too ill-conditioned.
                # We have degenerate vectors in our active set.
                # We'll 'drop for good' the last regressor added.

                # Note: this case is very rare. It is no longer triggered by
                # the test suite. The `equality_tolerance` margin added in 0.16
                # to get early stopping to work consistently on all versions of
                # Python including 32 bit Python under Windows seems to make it
                # very difficult to trigger the 'drop for good' strategy.
                warnings.warn('Regressors in active set degenerate. '
                              'Dropping a regressor, after %i iterations, '
                              'i.e. alpha=%.3e, '
                              'with an active set of %i regressors, and '
                              'the smallest cholesky pivot element being %.3e.'
                              ' Reduce max_iter or increase eps parameters.'
                              % (n_iter, alpha, n_active, diag),
                              ConvergenceWarning)

                # XXX: need to figure a 'drop for good' way
                Cov = Cov_not_shortened
                Cov[0] = 0
                Cov[C_idx], Cov[0] = swap(Cov[C_idx], Cov[0])
                continue

            active.append(indices[n_active])
            n_active += 1

            if verbose > 1:
                print("%s\t\t%s\t\t%s\t\t%s\t\t%s" % (n_iter, active[-1], '',
                                                      n_active, C))

        if method == 'lasso' and n_iter > 0 and prev_alpha[0] < alpha[0]:
            # alpha is increasing. This is because the updates of Cov are
            # bringing in too much numerical error that is greater than
            # than the remaining correlation with the
            # regressors. Time to bail out
            warnings.warn('Early stopping the lars path, as the residues '
                          'are small and the current value of alpha is no '
                          'longer well controlled. %i iterations, alpha=%.3e, '
                          'previous alpha=%.3e, with an active set of %i '
                          'regressors.'
                          % (n_iter, alpha, prev_alpha, n_active),
                          ConvergenceWarning)
            break

        # least squares solution
        least_squares, _ = solve_cholesky(L[:n_active, :n_active],
                                          sign_active[:n_active],
                                          lower=True)

        if least_squares.size == 1 and least_squares == 0:
            # This happens because sign_active[:n_active] = 0
            least_squares[...] = 1
            AA = 1.
        else:
            # is this really needed ?
            AA = 1. / np.sqrt(np.sum(least_squares * sign_active[:n_active]))

            if not np.isfinite(AA):
                # L is too ill-conditioned
                i = 0
                L_ = L[:n_active, :n_active].copy()
                while not np.isfinite(AA):
                    L_.flat[::n_active + 1] += (2 ** i) * eps
                    least_squares, _ = solve_cholesky(
                        L_, sign_active[:n_active], lower=True)
                    tmp = max(np.sum(least_squares * sign_active[:n_active]),
                              eps)
                    AA = 1. / np.sqrt(tmp)
                    i += 1
            least_squares *= AA

        if Gram is None:
            # equiangular direction of variables in the active set
            eq_dir = np.dot(X.T[:n_active].T, least_squares)
            # correlation between each unactive variables and
            # eqiangular vector
            corr_eq_dir = np.dot(X.T[n_active:], eq_dir)
        else:
            # if huge number of features, this takes 50% of time, I
            # think could be avoided if we just update it using an
            # orthogonal (QR) decomposition of X
            corr_eq_dir = np.dot(Gram[:n_active, n_active:].T,
                                 least_squares)

        g1 = arrayfuncs.min_pos((C - Cov) / (AA - corr_eq_dir + tiny32))
        if positive:
            gamma_ = min(g1, C / AA)
        else:
            g2 = arrayfuncs.min_pos((C + Cov) / (AA + corr_eq_dir + tiny32))
            gamma_ = min(g1, g2, C / AA)

        # TODO: better names for these variables: z
        drop = False
        z = -coef[active] / (least_squares + tiny32)
        z_pos = arrayfuncs.min_pos(z)
        if z_pos < gamma_:
            # some coefficients have changed sign
            idx = np.where(z == z_pos)[0][::-1]

            # update the sign, important for LAR
            sign_active[idx] = -sign_active[idx]

            if method == 'lasso':
                gamma_ = z_pos
            drop = True

        n_iter += 1

        if return_path:
            if n_iter >= coefs.shape[0]:
                del coef, alpha, prev_alpha, prev_coef
                # resize the coefs and alphas array
                add_features = 2 * max(1, (max_features - n_active))
                coefs = np.resize(coefs, (n_iter + add_features, n_features))
                coefs[-add_features:] = 0
                alphas = np.resize(alphas, n_iter + add_features)
                alphas[-add_features:] = 0
            coef = coefs[n_iter]
            prev_coef = coefs[n_iter - 1]
        else:
            # mimic the effect of incrementing n_iter on the array references
            prev_coef = coef
            prev_alpha[0] = alpha[0]
            coef = np.zeros_like(coef)

        coef[active] = prev_coef[active] + gamma_ * least_squares

        # update correlations
        Cov -= gamma_ * corr_eq_dir

        # See if any coefficient has changed sign
        if drop and method == 'lasso':

            # handle the case when idx is not length of 1
            for ii in idx:
                arrayfuncs.cholesky_delete(L[:n_active, :n_active], ii)

            n_active -= 1
            # handle the case when idx is not length of 1
            drop_idx = [active.pop(ii) for ii in idx]

            if Gram is None:
                # propagate dropped variable
                for ii in idx:
                    for i in range(ii, n_active):
                        X.T[i], X.T[i + 1] = swap(X.T[i], X.T[i + 1])
                        # yeah this is stupid
                        indices[i], indices[i + 1] = indices[i + 1], indices[i]

                # TODO: this could be updated
                residual = y - np.dot(X[:, :n_active], coef[active])
                temp = np.dot(X.T[n_active], residual)

                Cov = np.r_[temp, Cov]
            else:
                for ii in idx:
                    for i in range(ii, n_active):
                        indices[i], indices[i + 1] = indices[i + 1], indices[i]
                        Gram[i], Gram[i + 1] = swap(Gram[i], Gram[i + 1])
                        Gram[:, i], Gram[:, i + 1] = swap(Gram[:, i],
                                                          Gram[:, i + 1])

                # Cov_n = Cov_j + x_j * X + increment(betas) TODO:
                # will this still work with multiple drops ?

                # recompute covariance. Probably could be done better
                # wrong as Xy is not swapped with the rest of variables

                # TODO: this could be updated
                temp = Cov_copy[drop_idx] - np.dot(Gram_copy[drop_idx], coef)
                Cov = np.r_[temp, Cov]

            sign_active = np.delete(sign_active, idx)
            sign_active = np.append(sign_active, 0.)  # just to maintain size
            if verbose > 1:
                print("%s\t\t%s\t\t%s\t\t%s\t\t%s" % (n_iter, '', drop_idx,
                                                      n_active, abs(temp)))

    if return_path:
        # resize coefs in case of early stop
        alphas = alphas[:n_iter + 1]
        coefs = coefs[:n_iter + 1]

        if return_n_iter:
            return alphas, active, coefs.T, n_iter
        else:
            return alphas, active, coefs.T
    else:
        if return_n_iter:
            return alpha, active, coef, n_iter
        else:
            return alpha, active, coef


###############################################################################
# Estimator classes

class Lars(MultiOutputMixin, RegressorMixin, LinearModel):
    """Least Angle Regression model a.k.a. LAR

    Read more in the :ref:`User Guide <least_angle_regression>`.

    Parameters
    ----------
    fit_intercept : bool, default=True
        Whether to calculate the intercept for this model. If set
        to false, no intercept will be used in calculations
        (i.e. data is expected to be centered).

    verbose : bool or int, default=False
        Sets the verbosity amount

    normalize : bool, default=True
        This parameter is ignored when ``fit_intercept`` is set to False.
        If True, the regressors X will be normalized before regression by
        subtracting the mean and dividing by the l2-norm.
        If you wish to standardize, please use
        :class:`sklearn.preprocessing.StandardScaler` before calling ``fit``
        on an estimator with ``normalize=False``.

    precompute : bool, 'auto' or array-like , default='auto'
        Whether to use a precomputed Gram matrix to speed up
        calculations. If set to ``'auto'`` let us decide. The Gram
        matrix can also be passed as argument.

    n_nonzero_coefs : int, default=500
        Target number of non-zero coefficients. Use ``np.inf`` for no limit.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. Unlike the ``tol`` parameter in some iterative
        optimization-based algorithms, this parameter does not control
        the tolerance of the optimization.
        By default, ``np.finfo(np.float).eps`` is used.

    copy_X : bool, default=True
        If ``True``, X will be copied; else, it may be overwritten.

    fit_path : bool, default=True
        If True the full path is stored in the ``coef_path_`` attribute.
        If you compute the solution for a large problem or many targets,
        setting ``fit_path`` to ``False`` will lead to a speedup, especially
        with a small alpha.

    jitter : float, default=None
        Upper bound on a uniform noise parameter to be added to the
        `y` values, to satisfy the model's assumption of
        one-at-a-time computations. Might help with stability.

    random_state : int, RandomState instance or None (default)
        Determines random number generation for jittering. Pass an int
        for reproducible output across multiple function calls.
        See :term:`Glossary <random_state>`. Ignored if `jitter` is None.

    Attributes
    ----------
    alphas_ : array-like of shape (n_alphas + 1,) | list of n_targets such \
            arrays
        Maximum of covariances (in absolute value) at each iteration. \
        ``n_alphas`` is either ``n_nonzero_coefs`` or ``n_features``, \
        whichever is smaller.

    active_ : list, length = n_alphas | list of n_targets such lists
        Indices of active variables at the end of the path.

    coef_path_ : array-like of shape (n_features, n_alphas + 1) \
        | list of n_targets such arrays
        The varying values of the coefficients along the path. It is not
        present if the ``fit_path`` parameter is ``False``.

    coef_ : array-like of shape (n_features,) or (n_targets, n_features)
        Parameter vector (w in the formulation formula).

    intercept_ : float or array-like of shape (n_targets,)
        Independent term in decision function.

    n_iter_ : array-like or int
        The number of iterations taken by lars_path to find the
        grid of alphas for each target.

    Examples
    --------
    >>> from sklearn import linear_model
    >>> reg = linear_model.Lars(n_nonzero_coefs=1)
    >>> reg.fit([[-1, 1], [0, 0], [1, 1]], [-1.1111, 0, -1.1111])
    Lars(n_nonzero_coefs=1)
    >>> print(reg.coef_)
    [ 0. -1.11...]

    See also
    --------
    lars_path, LarsCV
    sklearn.decomposition.sparse_encode

    """
    method = 'lar'
    positive = False

    @_deprecate_positional_args
    def __init__(self, *, fit_intercept=True, verbose=False, normalize=True,
                 precompute='auto', n_nonzero_coefs=500,
                 eps=np.finfo(np.float).eps, copy_X=True, fit_path=True,
                 jitter=None, random_state=None):
        self.fit_intercept = fit_intercept
        self.verbose = verbose
        self.normalize = normalize
        self.precompute = precompute
        self.n_nonzero_coefs = n_nonzero_coefs
        self.eps = eps
        self.copy_X = copy_X
        self.fit_path = fit_path
        self.jitter = jitter
        self.random_state = random_state

    @staticmethod
    def _get_gram(precompute, X, y):
        if (not hasattr(precompute, '__array__')) and (
                (precompute is True) or
                (precompute == 'auto' and X.shape[0] > X.shape[1]) or
                (precompute == 'auto' and y.shape[1] > 1)):
            precompute = np.dot(X.T, X)

        return precompute

    def _fit(self, X, y, max_iter, alpha, fit_path, Xy=None):
        """Auxiliary method to fit the model using X, y as training data"""
        n_features = X.shape[1]

        X, y, X_offset, y_offset, X_scale = self._preprocess_data(
            X, y, self.fit_intercept, self.normalize, self.copy_X)

        if y.ndim == 1:
            y = y[:, np.newaxis]

        n_targets = y.shape[1]

        Gram = self._get_gram(self.precompute, X, y)

        self.alphas_ = []
        self.n_iter_ = []
        self.coef_ = np.empty((n_targets, n_features))

        if fit_path:
            self.active_ = []
            self.coef_path_ = []
            for k in range(n_targets):
                this_Xy = None if Xy is None else Xy[:, k]
                alphas, active, coef_path, n_iter_ = lars_path(
                    X, y[:, k], Gram=Gram, Xy=this_Xy, copy_X=self.copy_X,
                    copy_Gram=True, alpha_min=alpha, method=self.method,
                    verbose=max(0, self.verbose - 1), max_iter=max_iter,
                    eps=self.eps, return_path=True,
                    return_n_iter=True, positive=self.positive)
                self.alphas_.append(alphas)
                self.active_.append(active)
                self.n_iter_.append(n_iter_)
                self.coef_path_.append(coef_path)
                self.coef_[k] = coef_path[:, -1]

            if n_targets == 1:
                self.alphas_, self.active_, self.coef_path_, self.coef_ = [
                    a[0] for a in (self.alphas_, self.active_, self.coef_path_,
                                   self.coef_)]
                self.n_iter_ = self.n_iter_[0]
        else:
            for k in range(n_targets):
                this_Xy = None if Xy is None else Xy[:, k]
                alphas, _, self.coef_[k], n_iter_ = lars_path(
                    X, y[:, k], Gram=Gram, Xy=this_Xy, copy_X=self.copy_X,
                    copy_Gram=True, alpha_min=alpha, method=self.method,
                    verbose=max(0, self.verbose - 1), max_iter=max_iter,
                    eps=self.eps, return_path=False, return_n_iter=True,
                    positive=self.positive)
                self.alphas_.append(alphas)
                self.n_iter_.append(n_iter_)
            if n_targets == 1:
                self.alphas_ = self.alphas_[0]
                self.n_iter_ = self.n_iter_[0]

        self._set_intercept(X_offset, y_offset, X_scale)
        return self

    def fit(self, X, y, Xy=None):
        """Fit the model using X, y as training data.

        Parameters
        ----------
        X : array-like of shape (n_samples, n_features)
            Training data.

        y : array-like of shape (n_samples,) or (n_samples, n_targets)
            Target values.

        Xy : array-like of shape (n_samples,) or (n_samples, n_targets), \
                default=None
            Xy = np.dot(X.T, y) that can be precomputed. It is useful
            only when the Gram matrix is precomputed.

        Returns
        -------
        self : object
            returns an instance of self.
        """
        X, y = self._validate_data(X, y, y_numeric=True, multi_output=True)

        alpha = getattr(self, 'alpha', 0.)
        if hasattr(self, 'n_nonzero_coefs'):
            alpha = 0.  # n_nonzero_coefs parametrization takes priority
            max_iter = self.n_nonzero_coefs
        else:
            max_iter = self.max_iter

        if self.jitter is not None:
            rng = check_random_state(self.random_state)

            noise = rng.uniform(high=self.jitter, size=len(y))
            y = y + noise

        self._fit(X, y, max_iter=max_iter, alpha=alpha, fit_path=self.fit_path,
                  Xy=Xy)

        return self


class LassoLars(Lars):
    """Lasso model fit with Least Angle Regression a.k.a. Lars

    It is a Linear Model trained with an L1 prior as regularizer.

    The optimization objective for Lasso is::

    (1 / (2 * n_samples)) * ||y - Xw||^2_2 + alpha * ||w||_1

    Read more in the :ref:`User Guide <least_angle_regression>`.

    Parameters
    ----------
    alpha : float, default=1.0
        Constant that multiplies the penalty term. Defaults to 1.0.
        ``alpha = 0`` is equivalent to an ordinary least square, solved
        by :class:`LinearRegression`. For numerical reasons, using
        ``alpha = 0`` with the LassoLars object is not advised and you
        should prefer the LinearRegression object.

    fit_intercept : bool, default=True
        whether to calculate the intercept for this model. If set
        to false, no intercept will be used in calculations
        (i.e. data is expected to be centered).

    verbose : bool or int, default=False
        Sets the verbosity amount

    normalize : bool, default=True
        This parameter is ignored when ``fit_intercept`` is set to False.
        If True, the regressors X will be normalized before regression by
        subtracting the mean and dividing by the l2-norm.
        If you wish to standardize, please use
        :class:`sklearn.preprocessing.StandardScaler` before calling ``fit``
        on an estimator with ``normalize=False``.

    precompute : bool, 'auto' or array-like, default='auto'
        Whether to use a precomputed Gram matrix to speed up
        calculations. If set to ``'auto'`` let us decide. The Gram
        matrix can also be passed as argument.

    max_iter : int, default=500
        Maximum number of iterations to perform.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. Unlike the ``tol`` parameter in some iterative
        optimization-based algorithms, this parameter does not control
        the tolerance of the optimization.
        By default, ``np.finfo(np.float).eps`` is used.

    copy_X : bool, default=True
        If True, X will be copied; else, it may be overwritten.

    fit_path : bool, default=True
        If ``True`` the full path is stored in the ``coef_path_`` attribute.
        If you compute the solution for a large problem or many targets,
        setting ``fit_path`` to ``False`` will lead to a speedup, especially
        with a small alpha.

    positive : bool, default=False
        Restrict coefficients to be >= 0. Be aware that you might want to
        remove fit_intercept which is set True by default.
        Under the positive restriction the model coefficients will not converge
        to the ordinary-least-squares solution for small values of alpha.
        Only coefficients up to the smallest alpha value (``alphas_[alphas_ >
        0.].min()`` when fit_path=True) reached by the stepwise Lars-Lasso
        algorithm are typically in congruence with the solution of the
        coordinate descent Lasso estimator.

    jitter : float, default=None
        Upper bound on a uniform noise parameter to be added to the
        `y` values, to satisfy the model's assumption of
        one-at-a-time computations. Might help with stability.

    random_state : int, RandomState instance or None (default)
        Determines random number generation for jittering. Pass an int
        for reproducible output across multiple function calls.
        See :term:`Glossary <random_state>`. Ignored if `jitter` is None.

    Attributes
    ----------
    alphas_ : array-like of shape (n_alphas + 1,) | list of n_targets such \
            arrays
        Maximum of covariances (in absolute value) at each iteration. \
        ``n_alphas`` is either ``max_iter``, ``n_features``, or the number of \
        nodes in the path with correlation greater than ``alpha``, whichever \
        is smaller.

    active_ : list, length = n_alphas | list of n_targets such lists
        Indices of active variables at the end of the path.

    coef_path_ : array-like of shape (n_features, n_alphas + 1) or list
        If a list is passed it's expected to be one of n_targets such arrays.
        The varying values of the coefficients along the path. It is not
        present if the ``fit_path`` parameter is ``False``.

    coef_ : array-like of shape (n_features,) or (n_targets, n_features)
        Parameter vector (w in the formulation formula).

    intercept_ : float or array-like of shape (n_targets,)
        Independent term in decision function.

    n_iter_ : array-like or int.
        The number of iterations taken by lars_path to find the
        grid of alphas for each target.

    Examples
    --------
    >>> from sklearn import linear_model
    >>> reg = linear_model.LassoLars(alpha=0.01)
    >>> reg.fit([[-1, 1], [0, 0], [1, 1]], [-1, 0, -1])
    LassoLars(alpha=0.01)
    >>> print(reg.coef_)
    [ 0.         -0.963257...]

    See also
    --------
    lars_path
    lasso_path
    Lasso
    LassoCV
    LassoLarsCV
    LassoLarsIC
    sklearn.decomposition.sparse_encode

    """
    method = 'lasso'

    @_deprecate_positional_args
    def __init__(self, alpha=1.0, *, fit_intercept=True, verbose=False,
                 normalize=True, precompute='auto', max_iter=500,
                 eps=np.finfo(np.float).eps, copy_X=True, fit_path=True,
                 positive=False, jitter=None, random_state=None):
        self.alpha = alpha
        self.fit_intercept = fit_intercept
        self.max_iter = max_iter
        self.verbose = verbose
        self.normalize = normalize
        self.positive = positive
        self.precompute = precompute
        self.copy_X = copy_X
        self.eps = eps
        self.fit_path = fit_path
        self.jitter = jitter
        self.random_state = random_state


###############################################################################
# Cross-validated estimator classes

def _check_copy_and_writeable(array, copy=False):
    if copy or not array.flags.writeable:
        return array.copy()
    return array


def _lars_path_residues(X_train, y_train, X_test, y_test, Gram=None,
                        copy=True, method='lars', verbose=False,
                        fit_intercept=True, normalize=True, max_iter=500,
                        eps=np.finfo(np.float).eps, positive=False):
    """Compute the residues on left-out data for a full LARS path

    Parameters
    -----------
    X_train : array-like of shape (n_samples, n_features)
        The data to fit the LARS on

    y_train : array-like of shape (n_samples,)
        The target variable to fit LARS on

    X_test : array-like of shape (n_samples, n_features)
        The data to compute the residues on

    y_test : array-like of shape (n_samples,)
        The target variable to compute the residues on

    Gram : None, 'auto' or array-like of shape (n_features, n_features), \
            default=None
        Precomputed Gram matrix (X' * X), if ``'auto'``, the Gram
        matrix is precomputed from the given X, if there are more samples
        than features

    copy : bool, default=True
        Whether X_train, X_test, y_train and y_test should be copied;
        if False, they may be overwritten.

    method : {'lar' , 'lasso'}, default='lar'
        Specifies the returned model. Select ``'lar'`` for Least Angle
        Regression, ``'lasso'`` for the Lasso.

    verbose : bool or int, default=False
        Sets the amount of verbosity

    fit_intercept : bool, default=True
        whether to calculate the intercept for this model. If set
        to false, no intercept will be used in calculations
        (i.e. data is expected to be centered).

    positive : bool, default=False
        Restrict coefficients to be >= 0. Be aware that you might want to
        remove fit_intercept which is set True by default.
        See reservations for using this option in combination with method
        'lasso' for expected small values of alpha in the doc of LassoLarsCV
        and LassoLarsIC.

    normalize : bool, default=True
        This parameter is ignored when ``fit_intercept`` is set to False.
        If True, the regressors X will be normalized before regression by
        subtracting the mean and dividing by the l2-norm.
        If you wish to standardize, please use
        :class:`sklearn.preprocessing.StandardScaler` before calling ``fit``
        on an estimator with ``normalize=False``.

    max_iter : int, default=500
        Maximum number of iterations to perform.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. Unlike the ``tol`` parameter in some iterative
        optimization-based algorithms, this parameter does not control
        the tolerance of the optimization.
        By default, ``np.finfo(np.float).eps`` is used


    Returns
    --------
    alphas : array-like of shape (n_alphas,)
        Maximum of covariances (in absolute value) at each iteration.
        ``n_alphas`` is either ``max_iter`` or ``n_features``, whichever
        is smaller.

    active : list
        Indices of active variables at the end of the path.

    coefs : array-like of shape (n_features, n_alphas)
        Coefficients along the path

    residues : array-like of shape (n_alphas, n_samples)
        Residues of the prediction on the test data
    """
    X_train = _check_copy_and_writeable(X_train, copy)
    y_train = _check_copy_and_writeable(y_train, copy)
    X_test = _check_copy_and_writeable(X_test, copy)
    y_test = _check_copy_and_writeable(y_test, copy)

    if fit_intercept:
        X_mean = X_train.mean(axis=0)
        X_train -= X_mean
        X_test -= X_mean
        y_mean = y_train.mean(axis=0)
        y_train = as_float_array(y_train, copy=False)
        y_train -= y_mean
        y_test = as_float_array(y_test, copy=False)
        y_test -= y_mean

    if normalize:
        norms = np.sqrt(np.sum(X_train ** 2, axis=0))
        nonzeros = np.flatnonzero(norms)
        X_train[:, nonzeros] /= norms[nonzeros]

    alphas, active, coefs = lars_path(
        X_train, y_train, Gram=Gram, copy_X=False, copy_Gram=False,
        method=method, verbose=max(0, verbose - 1), max_iter=max_iter, eps=eps,
        positive=positive)
    if normalize:
        coefs[nonzeros] /= norms[nonzeros][:, np.newaxis]
    residues = np.dot(X_test, coefs) - y_test[:, np.newaxis]
    return alphas, active, coefs, residues.T


class LarsCV(Lars):
    """Cross-validated Least Angle Regression model.

    See glossary entry for :term:`cross-validation estimator`.

    Read more in the :ref:`User Guide <least_angle_regression>`.

    Parameters
    ----------
    fit_intercept : bool, default=True
        whether to calculate the intercept for this model. If set
        to false, no intercept will be used in calculations
        (i.e. data is expected to be centered).

    verbose : bool or int, default=False
        Sets the verbosity amount

    max_iter : int, default=500
        Maximum number of iterations to perform.

    normalize : bool, default=True
        This parameter is ignored when ``fit_intercept`` is set to False.
        If True, the regressors X will be normalized before regression by
        subtracting the mean and dividing by the l2-norm.
        If you wish to standardize, please use
        :class:`sklearn.preprocessing.StandardScaler` before calling ``fit``
        on an estimator with ``normalize=False``.

    precompute : bool, 'auto' or array-like , default='auto'
        Whether to use a precomputed Gram matrix to speed up
        calculations. If set to ``'auto'`` let us decide. The Gram matrix
        cannot be passed as argument since we will use only subsets of X.

    cv : int, cross-validation generator or an iterable, default=None
        Determines the cross-validation splitting strategy.
        Possible inputs for cv are:

        - None, to use the default 5-fold cross-validation,
        - integer, to specify the number of folds.
        - :term:`CV splitter`,
        - An iterable yielding (train, test) splits as arrays of indices.

        For integer/None inputs, :class:`KFold` is used.

        Refer :ref:`User Guide <cross_validation>` for the various
        cross-validation strategies that can be used here.

        .. versionchanged:: 0.22
            ``cv`` default value if None changed from 3-fold to 5-fold.

    max_n_alphas : int, default=1000
        The maximum number of points on the path used to compute the
        residuals in the cross-validation

    n_jobs : int or None, default=None
        Number of CPUs to use during the cross validation.
        ``None`` means 1 unless in a :obj:`joblib.parallel_backend` context.
        ``-1`` means using all processors. See :term:`Glossary <n_jobs>`
        for more details.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. By default, ``np.finfo(np.float).eps`` is used.

    copy_X : bool, default=True
        If ``True``, X will be copied; else, it may be overwritten.

    Attributes
    ----------
    coef_ : array-like of shape (n_features,)
        parameter vector (w in the formulation formula)

    intercept_ : float
        independent term in decision function

    coef_path_ : array-like of shape (n_features, n_alphas)
        the varying values of the coefficients along the path

    alpha_ : float
        the estimated regularization parameter alpha

    alphas_ : array-like of shape (n_alphas,)
        the different values of alpha along the path

    cv_alphas_ : array-like of shape (n_cv_alphas,)
        all the values of alpha along the path for the different folds

    mse_path_ : array-like of shape (n_folds, n_cv_alphas)
        the mean square error on left-out for each fold along the path
        (alpha values given by ``cv_alphas``)

    n_iter_ : array-like or int
        the number of iterations run by Lars with the optimal alpha.

    Examples
    --------
    >>> from sklearn.linear_model import LarsCV
    >>> from sklearn.datasets import make_regression
    >>> X, y = make_regression(n_samples=200, noise=4.0, random_state=0)
    >>> reg = LarsCV(cv=5).fit(X, y)
    >>> reg.score(X, y)
    0.9996...
    >>> reg.alpha_
    0.0254...
    >>> reg.predict(X[:1,])
    array([154.0842...])

    See also
    --------
    lars_path, LassoLars, LassoLarsCV
    """

    method = 'lar'

    @_deprecate_positional_args
    def __init__(self, *, fit_intercept=True, verbose=False, max_iter=500,
                 normalize=True, precompute='auto', cv=None,
                 max_n_alphas=1000, n_jobs=None, eps=np.finfo(np.float).eps,
                 copy_X=True):
        self.max_iter = max_iter
        self.cv = cv
        self.max_n_alphas = max_n_alphas
        self.n_jobs = n_jobs
        super().__init__(fit_intercept=fit_intercept,
                         verbose=verbose, normalize=normalize,
                         precompute=precompute,
                         n_nonzero_coefs=500,
                         eps=eps, copy_X=copy_X, fit_path=True)

    def _more_tags(self):
        return {'multioutput': False}

    def fit(self, X, y):
        """Fit the model using X, y as training data.

        Parameters
        ----------
        X : array-like of shape (n_samples, n_features)
            Training data.

        y : array-like of shape (n_samples,)
            Target values.

        Returns
        -------
        self : object
            returns an instance of self.
        """
        X, y = self._validate_data(X, y, y_numeric=True)
        X = as_float_array(X, copy=self.copy_X)
        y = as_float_array(y, copy=self.copy_X)

        # init cross-validation generator
        cv = check_cv(self.cv, classifier=False)

        # As we use cross-validation, the Gram matrix is not precomputed here
        Gram = self.precompute
        if hasattr(Gram, '__array__'):
            warnings.warn('Parameter "precompute" cannot be an array in '
                          '%s. Automatically switch to "auto" instead.'
                          % self.__class__.__name__)
            Gram = 'auto'

        cv_paths = Parallel(n_jobs=self.n_jobs, verbose=self.verbose)(
            delayed(_lars_path_residues)(
                X[train], y[train], X[test], y[test], Gram=Gram, copy=False,
                method=self.method, verbose=max(0, self.verbose - 1),
                normalize=self.normalize, fit_intercept=self.fit_intercept,
                max_iter=self.max_iter, eps=self.eps, positive=self.positive)
            for train, test in cv.split(X, y))
        all_alphas = np.concatenate(list(zip(*cv_paths))[0])
        # Unique also sorts
        all_alphas = np.unique(all_alphas)
        # Take at most max_n_alphas values
        stride = int(max(1, int(len(all_alphas) / float(self.max_n_alphas))))
        all_alphas = all_alphas[::stride]

        mse_path = np.empty((len(all_alphas), len(cv_paths)))
        for index, (alphas, _, _, residues) in enumerate(cv_paths):
            alphas = alphas[::-1]
            residues = residues[::-1]
            if alphas[0] != 0:
                alphas = np.r_[0, alphas]
                residues = np.r_[residues[0, np.newaxis], residues]
            if alphas[-1] != all_alphas[-1]:
                alphas = np.r_[alphas, all_alphas[-1]]
                residues = np.r_[residues, residues[-1, np.newaxis]]
            this_residues = interpolate.interp1d(alphas,
                                                 residues,
                                                 axis=0)(all_alphas)
            this_residues **= 2
            mse_path[:, index] = np.mean(this_residues, axis=-1)

        mask = np.all(np.isfinite(mse_path), axis=-1)
        all_alphas = all_alphas[mask]
        mse_path = mse_path[mask]
        # Select the alpha that minimizes left-out error
        i_best_alpha = np.argmin(mse_path.mean(axis=-1))
        best_alpha = all_alphas[i_best_alpha]

        # Store our parameters
        self.alpha_ = best_alpha
        self.cv_alphas_ = all_alphas
        self.mse_path_ = mse_path

        # Now compute the full model
        # it will call a lasso internally when self if LassoLarsCV
        # as self.method == 'lasso'
        self._fit(X, y, max_iter=self.max_iter, alpha=best_alpha,
                  Xy=None, fit_path=True)
        return self


class LassoLarsCV(LarsCV):
    """Cross-validated Lasso, using the LARS algorithm.

    See glossary entry for :term:`cross-validation estimator`.

    The optimization objective for Lasso is::

    (1 / (2 * n_samples)) * ||y - Xw||^2_2 + alpha * ||w||_1

    Read more in the :ref:`User Guide <least_angle_regression>`.

    Parameters
    ----------
    fit_intercept : bool, default=True
        whether to calculate the intercept for this model. If set
        to false, no intercept will be used in calculations
        (i.e. data is expected to be centered).

    verbose : bool or int, default=False
        Sets the verbosity amount

    max_iter : int, default=500
        Maximum number of iterations to perform.

    normalize : bool, default=True
        This parameter is ignored when ``fit_intercept`` is set to False.
        If True, the regressors X will be normalized before regression by
        subtracting the mean and dividing by the l2-norm.
        If you wish to standardize, please use
        :class:`sklearn.preprocessing.StandardScaler` before calling ``fit``
        on an estimator with ``normalize=False``.

    precompute : bool or 'auto' , default='auto'
        Whether to use a precomputed Gram matrix to speed up
        calculations. If set to ``'auto'`` let us decide. The Gram matrix
        cannot be passed as argument since we will use only subsets of X.

    cv : int, cross-validation generator or an iterable, default=None
        Determines the cross-validation splitting strategy.
        Possible inputs for cv are:

        - None, to use the default 5-fold cross-validation,
        - integer, to specify the number of folds.
        - :term:`CV splitter`,
        - An iterable yielding (train, test) splits as arrays of indices.

        For integer/None inputs, :class:`KFold` is used.

        Refer :ref:`User Guide <cross_validation>` for the various
        cross-validation strategies that can be used here.

        .. versionchanged:: 0.22
            ``cv`` default value if None changed from 3-fold to 5-fold.

    max_n_alphas : int, default=1000
        The maximum number of points on the path used to compute the
        residuals in the cross-validation

    n_jobs : int or None, default=None
        Number of CPUs to use during the cross validation.
        ``None`` means 1 unless in a :obj:`joblib.parallel_backend` context.
        ``-1`` means using all processors. See :term:`Glossary <n_jobs>`
        for more details.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. By default, ``np.finfo(np.float).eps`` is used.

    copy_X : bool, default=True
        If True, X will be copied; else, it may be overwritten.

    positive : bool, default=False
        Restrict coefficients to be >= 0. Be aware that you might want to
        remove fit_intercept which is set True by default.
        Under the positive restriction the model coefficients do not converge
        to the ordinary-least-squares solution for small values of alpha.
        Only coefficients up to the smallest alpha value (``alphas_[alphas_ >
        0.].min()`` when fit_path=True) reached by the stepwise Lars-Lasso
        algorithm are typically in congruence with the solution of the
        coordinate descent Lasso estimator.
        As a consequence using LassoLarsCV only makes sense for problems where
        a sparse solution is expected and/or reached.

    Attributes
    ----------
    coef_ : array-like of shape (n_features,)
        parameter vector (w in the formulation formula)

    intercept_ : float
        independent term in decision function.

    coef_path_ : array-like of shape (n_features, n_alphas)
        the varying values of the coefficients along the path

    alpha_ : float
        the estimated regularization parameter alpha

    alphas_ : array-like of shape (n_alphas,)
        the different values of alpha along the path

    cv_alphas_ : array-like of shape (n_cv_alphas,)
        all the values of alpha along the path for the different folds

    mse_path_ : array-like of shape (n_folds, n_cv_alphas)
        the mean square error on left-out for each fold along the path
        (alpha values given by ``cv_alphas``)

    n_iter_ : array-like or int
        the number of iterations run by Lars with the optimal alpha.

    Examples
    --------
    >>> from sklearn.linear_model import LassoLarsCV
    >>> from sklearn.datasets import make_regression
    >>> X, y = make_regression(noise=4.0, random_state=0)
    >>> reg = LassoLarsCV(cv=5).fit(X, y)
    >>> reg.score(X, y)
    0.9992...
    >>> reg.alpha_
    0.0484...
    >>> reg.predict(X[:1,])
    array([-77.8723...])

    Notes
    -----

    The object solves the same problem as the LassoCV object. However,
    unlike the LassoCV, it find the relevant alphas values by itself.
    In general, because of this property, it will be more stable.
    However, it is more fragile to heavily multicollinear datasets.

    It is more efficient than the LassoCV if only a small number of
    features are selected compared to the total number, for instance if
    there are very few samples compared to the number of features.

    See also
    --------
    lars_path, LassoLars, LarsCV, LassoCV
    """

    method = 'lasso'

    @_deprecate_positional_args
    def __init__(self, *, fit_intercept=True, verbose=False, max_iter=500,
                 normalize=True, precompute='auto', cv=None,
                 max_n_alphas=1000, n_jobs=None, eps=np.finfo(np.float).eps,
                 copy_X=True, positive=False):
        self.fit_intercept = fit_intercept
        self.verbose = verbose
        self.max_iter = max_iter
        self.normalize = normalize
        self.precompute = precompute
        self.cv = cv
        self.max_n_alphas = max_n_alphas
        self.n_jobs = n_jobs
        self.eps = eps
        self.copy_X = copy_X
        self.positive = positive
        # XXX : we don't use super().__init__
        # to avoid setting n_nonzero_coefs


class LassoLarsIC(LassoLars):
    """Lasso model fit with Lars using BIC or AIC for model selection

    The optimization objective for Lasso is::

    (1 / (2 * n_samples)) * ||y - Xw||^2_2 + alpha * ||w||_1

    AIC is the Akaike information criterion and BIC is the Bayes
    Information criterion. Such criteria are useful to select the value
    of the regularization parameter by making a trade-off between the
    goodness of fit and the complexity of the model. A good model should
    explain well the data while being simple.

    Read more in the :ref:`User Guide <least_angle_regression>`.

    Parameters
    ----------
    criterion : {'bic' , 'aic'}, default='aic'
        The type of criterion to use.

    fit_intercept : bool, default=True
        whether to calculate the intercept for this model. If set
        to false, no intercept will be used in calculations
        (i.e. data is expected to be centered).

    verbose : bool or int, default=False
        Sets the verbosity amount

    normalize : bool, default=True
        This parameter is ignored when ``fit_intercept`` is set to False.
        If True, the regressors X will be normalized before regression by
        subtracting the mean and dividing by the l2-norm.
        If you wish to standardize, please use
        :class:`sklearn.preprocessing.StandardScaler` before calling ``fit``
        on an estimator with ``normalize=False``.

    precompute : bool, 'auto' or array-like, default='auto'
        Whether to use a precomputed Gram matrix to speed up
        calculations. If set to ``'auto'`` let us decide. The Gram
        matrix can also be passed as argument.

    max_iter : int, default=500
        Maximum number of iterations to perform. Can be used for
        early stopping.

    eps : float, optional
        The machine-precision regularization in the computation of the
        Cholesky diagonal factors. Increase this for very ill-conditioned
        systems. Unlike the ``tol`` parameter in some iterative
        optimization-based algorithms, this parameter does not control
        the tolerance of the optimization.
        By default, ``np.finfo(np.float).eps`` is used

    copy_X : bool, default=True
        If True, X will be copied; else, it may be overwritten.

    positive : bool, default=False
        Restrict coefficients to be >= 0. Be aware that you might want to
        remove fit_intercept which is set True by default.
        Under the positive restriction the model coefficients do not converge
        to the ordinary-least-squares solution for small values of alpha.
        Only coefficients up to the smallest alpha value (``alphas_[alphas_ >
        0.].min()`` when fit_path=True) reached by the stepwise Lars-Lasso
        algorithm are typically in congruence with the solution of the
        coordinate descent Lasso estimator.
        As a consequence using LassoLarsIC only makes sense for problems where
        a sparse solution is expected and/or reached.

    Attributes
    ----------
    coef_ : array-like of shape (n_features,)
        parameter vector (w in the formulation formula)

    intercept_ : float
        independent term in decision function.

    alpha_ : float
        the alpha parameter chosen by the information criterion

    n_iter_ : int
        number of iterations run by lars_path to find the grid of
        alphas.

    criterion_ : array-like of shape (n_alphas,)
        The value of the information criteria ('aic', 'bic') across all
        alphas. The alpha which has the smallest information criterion is
        chosen. This value is larger by a factor of ``n_samples`` compared to
        Eqns. 2.15 and 2.16 in (Zou et al, 2007).


    Examples
    --------
    >>> from sklearn import linear_model
    >>> reg = linear_model.LassoLarsIC(criterion='bic')
    >>> reg.fit([[-1, 1], [0, 0], [1, 1]], [-1.1111, 0, -1.1111])
    LassoLarsIC(criterion='bic')
    >>> print(reg.coef_)
    [ 0.  -1.11...]

    Notes
    -----
    The estimation of the number of degrees of freedom is given by:

    "On the degrees of freedom of the lasso"
    Hui Zou, Trevor Hastie, and Robert Tibshirani
    Ann. Statist. Volume 35, Number 5 (2007), 2173-2192.

    https://en.wikipedia.org/wiki/Akaike_information_criterion
    https://en.wikipedia.org/wiki/Bayesian_information_criterion

    See also
    --------
    lars_path, LassoLars, LassoLarsCV
    """
    @_deprecate_positional_args
    def __init__(self, criterion='aic', *, fit_intercept=True, verbose=False,
                 normalize=True, precompute='auto', max_iter=500,
                 eps=np.finfo(np.float).eps, copy_X=True, positive=False):
        self.criterion = criterion
        self.fit_intercept = fit_intercept
        self.positive = positive
        self.max_iter = max_iter
        self.verbose = verbose
        self.normalize = normalize
        self.copy_X = copy_X
        self.precompute = precompute
        self.eps = eps
        self.fit_path = True

    def _more_tags(self):
        return {'multioutput': False}

    def fit(self, X, y, copy_X=None):
        """Fit the model using X, y as training data.

        Parameters
        ----------
        X : array-like of shape (n_samples, n_features)
            training data.

        y : array-like of shape (n_samples,)
            target values. Will be cast to X's dtype if necessary

        copy_X : bool, default=None
            If provided, this parameter will override the choice
            of copy_X made at instance creation.
            If ``True``, X will be copied; else, it may be overwritten.

        Returns
        -------
        self : object
            returns an instance of self.
        """
        if copy_X is None:
            copy_X = self.copy_X
        X, y = self._validate_data(X, y, y_numeric=True)

        X, y, Xmean, ymean, Xstd = LinearModel._preprocess_data(
            X, y, self.fit_intercept, self.normalize, copy_X)
        max_iter = self.max_iter

        Gram = self.precompute

        alphas_, _, coef_path_, self.n_iter_ = lars_path(
            X, y, Gram=Gram, copy_X=copy_X, copy_Gram=True, alpha_min=0.0,
            method='lasso', verbose=self.verbose, max_iter=max_iter,
            eps=self.eps, return_n_iter=True, positive=self.positive)

        n_samples = X.shape[0]

        if self.criterion == 'aic':
            K = 2  # AIC
        elif self.criterion == 'bic':
            K = log(n_samples)  # BIC
        else:
            raise ValueError('criterion should be either bic or aic')

        R = y[:, np.newaxis] - np.dot(X, coef_path_)  # residuals
        mean_squared_error = np.mean(R ** 2, axis=0)
        sigma2 = np.var(y)

        df = np.zeros(coef_path_.shape[1], dtype=np.int)  # Degrees of freedom
        for k, coef in enumerate(coef_path_.T):
            mask = np.abs(coef) > np.finfo(coef.dtype).eps
            if not np.any(mask):
                continue
            # get the number of degrees of freedom equal to:
            # Xc = X[:, mask]
            # Trace(Xc * inv(Xc.T, Xc) * Xc.T) ie the number of non-zero coefs
            df[k] = np.sum(mask)

        self.alphas_ = alphas_
        eps64 = np.finfo('float64').eps
        self.criterion_ = (n_samples * mean_squared_error / (sigma2 + eps64) +
                           K * df)  # Eqns. 2.15--16 in (Zou et al, 2007)
        n_best = np.argmin(self.criterion_)

        self.alpha_ = alphas_[n_best]
        self.coef_ = coef_path_[:, n_best]
        self._set_intercept(Xmean, ymean, Xstd)
        return self
